Article pubs.acs.org/Organometallics
Synthesis, Spectroelectrochemical, and EPR Spectroscopic Studies of Mixed Bis(alkynyl)biferrocenes of the Type (LnMCC)(LnM′CC)bfc Manja Lohan,†,‡ Frédéric Justaud,† Heinrich Lang,*,‡ and Claude Lapinte*,† †
Sciences Chimiques de Rennes, Université de Rennes 1, UMR CNRS 6226, Campus de Beaulieu, F-35042 Rennes, France Fakultät für Naturwissenschaften, Institut für Chemie, Lehrstuhl für Anorganische Chemie, Technische Universität Chemnitz, Strasse der Nationen 62, 09111 Chemnitz, Germany
‡
S Supporting Information *
ABSTRACT: The synthesis and properties of a series of complexes containing bis(ethynyl)biferrocene as a bridge between different redoxactive group 8 metal fragments is described. These metal acetylide compounds of type (LnMCC)(LnM′CC)bfc (5a, LnM = Fe(Cp*)(η2-dppe), LnM′ = Ru(Cp)(Ph3P)2; 5b, LnM = Fe(Cp*)(η2-dppe), LnM′ = Os(Cp)(Ph3P)2; 5c, LnM = Ru(Cp)(Ph3P)2, LnM′ = Os(Cp)(Ph3P)2; bfc = biferrocene-1′,1‴-diyl, ((η5-C5H4)2Fe)2; dppe = 1,2-bis(diphenylphosphino)ethane, C2H4(PPh2)2; Cp = η5-C5H5, Cp* = η5-C5Me5) were prepared either by treatment of (HCC)(LnM′CC)bfc (LnM′ = Ru(Cp)(Ph3P)2 (4b), Os(Cp)(Ph3P)2 (4c)) with Fe(Cp*)(η2-dppe)Cl (2a) or by the reaction of 4c with Ru(Cp)(PPh3)2Cl (2b) in the presence of [H4N][PF6] and KOtBu, respectively. Compounds 5a−c show wellseparated reversible one-electron redox events in their cyclic voltammograms using [nBu4N][PF6] as supporting electrolyte in dichloromethane solutions. Absorption and vibrational spectroscopic studies were achieved for mixed-valence 5a−c[PF6] and 5a−c[PF6]2 by spectroelectrochemical methods (OTTLE), and in the case of the more robust Fe/Os system the higher oxidation states 5b[PF6]3 and 5b[PF6]4 were also characterized. Taken as a whole, our data indicate that direct electron transfer between the redox termini does not take place. Electron exchange results from dominant interactions between the redox termini and the proximal fc units (fc = Fe(η5-C5H4)2) of the bfc moiety and a weak but sizable interaction between the fc units. Furthermore, EPR spectroscopy of 5a−c[PF6] allowed the simultaneous observation of the EPR signatures of half-sandwich metal-centered radicals and biferrocenium-centered radicals. This feature strongly supports that a multistep electron exchange mechanism takes place between the MLn/M′Ln redox termini of this molecular array, with bridge-centered low-lying mediating states thermally populated even at 66 K. The g tensors of anisotropy (Δg = g∥ − g⊥) of the bis(ethynyl)biferrocenyl moiety ranging between 2.26 and 2.42 for 5a−c[PF6] are consistent with a slow electron exchange rate between the fc units and confirmed that these mixed-valence complexes belong to class II compounds as defined by Robin and Day.
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INTRODUCTION Organometallic molecules containing unsaturated carbon-rich chains end-capped by identical redox-active 17-valence-electron half-sandwich metal fragments have been well studied with regard to their electrochemical and/or spectroelectrochemical properties: i.e. (Cp)(PPh3)2Ru−(CC)2−Ru(PPh3)2(Cp), (Cp)(NO)(PPh3)Re−(CC) 2−Re(PPh3)(NO)(Cp), and (Cp*)(dppe)Fe−(CC) 2 -Fe(dppe)(Cp*).1−12 Rigid-rod structured molecules featuring redox-active terminal metallocenyl groups have also been extensively studied, since they represent model compounds for molecular wire molecules.9,13−17 In addition, a combination of sandwich and halfsandwich units bridged by linear alkynyl building blocks (CC)n (n = 0, 1, 2, 3, ...) or other carbon-rich connectivities, including 1,4-diethynyl- or 1,3,5-triethynylbenzene, are known, and their spectro-electrochemical behavior was investigated in detail.11,18−26 In particular, it was found that lengthening of the carbon chain (C2 to C8) resulted in an increase of the redox © XXXX American Chemical Society
potential of the end-grafted ferrocenyl moiety in, for example, Fc−(CC)n−WCp(CO)3 (Fc = Fe(η5-C5H4)(η5-C5H5); Cp = η5-C5H5; n = 1−4).27 Metallocenyls also can be incorporated in alkynyl chains, resulting in the formation of (LnMC C)(LnM′CC)fc compounds (fc = (η5-C5H4)2Fe; LnM = LnM′; LnM ≠ LnM′; 17-valence-electron half-sandwich or sandwich entities).1,2,21,28−31 These molecules were prepared to examine the transmission of electronic effects between the appropriate terminal transition metals on the carbon connectivities via the central fc unit. However, this redox-active group, interrupting the all-carbon chain, acts as an insulator rather than a transmitter when, for example, it is inserted into a butadi-1,3-yne group of archetypal molecular wires: for example, (Cp)(PPh 3 ) 2 Ru−(CC) 2 −Ru(PPh 3 ) 2 (Cp) forms ((Cp)(PPh3)2Ru−CC)2fc (4).21 Recently, Dong and Received: January 20, 2012
A
dx.doi.org/10.1021/om300050t | Organometallics XXXX, XXX, XXX−XXX
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Scheme 1. Synthesis of (LnMCC)(LnM′CC)bfc (5a−c) from (HCC)(LnM′CC)bfc (4)a
a
Key reagents and conditions: (i) [NH4][PF6] (1 equiv), thf/MeOH, 25 °C, 16 h; (ii) KOtBu (1 equiv), 25 °C, 5 h.
the dehydrohalogenation route shown in Scheme 1. We chose 4b,c as starting materials for our synthesis because (HCC)((Cp*)(η2-dppe)FeCC)bfc (4a) could not be obtained in pure form. Within such reactions metallavinylidene species are ubiquitously formed as intermediates;19,21,34−37 however, the isolation of these dicationic compounds failed, since they rapidly decompose under the reaction conditions applied. For this reason, we directly deprotonated the appropriate metallavinylidenes as they were formed using KOtBu as base. After the appropriate workup, the tetranuclear neutral complexes 5a−c could be isolated as orange solids in moderate yields (Experimental Section). These organometallics can be handled for a short period of time in air and are soluble in most common organic solvents. Solutions containing these compounds are more sensitive to air and moisture, and decomposition occurs in minutes. The progress of the complexation of 4b,c with 2a and of 4c with 2b to give 5a−c, respectively, could be monitored by IR spectroscopy, since the typical stretching frequencies of the HCC unit in 4b,c (νHC 3302 cm−1 and νCC 2107 cm−1) disappear during the course of the reaction and new νCC vibrations typical for LnMCC fragments appear. That indeed two different end-capped organometallic entities are present is confirmed by a second absorption or broadening of the already existing bands at νCC 2059, 2075 (5a), νCC 2060, 2077 (5b), and νCC 2075 cm−1 (5c). Neutral molecules 5a−c have been characterized by elemental analysis as well as by IR and NMR spectroscopy (1H, 13C{1H}, 31P{1H} NMR). In addition, the electrochemical and spectroscopical properties of 5a−c were determined using cyclic voltammetry (CV) and in situ UV−vis−near-IR and IR spectroscopy as well as EPR spectroscopy. The existence of tetranuclear 5a−c with different half-sandwich termini was confirmed by 31P{1H} NMR spectroscopy, since two resonance signal sets are observed as typical for these mixed heterometallic MM′Fe2 species (Experimental Section). The phosphorus resonance signal for the Fe(Cp*)(dppe) unit appears at 112 (5a) and 113 ppm (5b), while for the M(Cp)(Ph3P)2 moieties the phosphorus signals were observed at 14.2 (M = Os, 5b) and 64.1 ppm (M = Ru, 5a). In 5c the signals for the Os(Cp)(PPh3)2 and Ru(Cp)(PPh3)2 moieties were observed at 1.1 and 50.4 ppm, respectively, which is typical for these transition-metal building blocks.38−48 Like IR spectroscopy (vide supra), 31P{1H} NMR spectroscopy can also be used for monitoring the progress of the reactions. For example, changing from 2 to 5 is accompanied by a chemical
co-workers reported electronic communication in molecules of the type ((Cp)(η2-dppe)RuCC)2fc.28 It is also known that biferrocene-1′,1‴-diynes can act as linking groups for diverse organometallic termini, conveying intramolecular electronic interaction from one end to the other, whereby the strongest interaction occurs between the terminal metal center and the bfc unit.28,32,33 This prompted us to synthesize and study the electrochemical and spectroelectrochemical properties of mixed-transition-metal complexes featuring the bis(CC)biferrocenyl moiety as a connecting unit between redox-active termini, since only little is known about this family of compounds.1,2,30 One example is (Cp*)(η2-dppe)Fe−CC− bfc−CC−Fe(η2-dppe)Cp* (3a), showing that the mixedvalent species is in alignment with a class II molecule in the classification scheme of Robin and Day with an electron transfer involving intermediate states.32 We here report on (i) the consecutive preparation and characterization of tetrametallic (LnMCC)(LnM′CC)bfc assemblies (5a, LnM = FeCp*(η2-dppe), LnM′ = RuCp(PPh3)2; 5b, LnM = FeCp*(η2-dppe), LnM′ = OsCp(PPh3)2; 5c, LnM = RuCp(PPh3)2, LnM′ = OsCp(PPh3)2; bfc = biferrocene-1′,1‴-diyl, ((η5-C5H4)2Fe)2; dppe = 1,2-bis(diphenylphosphino)ethane; Cp = η5-C5H5, Cp* = η5-C5Me5), (ii) the redox behavior of these new complexes using cyclic voltammetry (CV), (iii) the in situ generation of the corresponding oxidized species 5a[PF6]n, 5c[PF6]n (n = 1, 2), and 5b[PF6]n (n = 1−4) by electrochemical oxidation and their characterization by IR spectroscopy, (iv) the analysis of the location of the spin density for the radical cations 5a−c[PF 6] investigated by EPR spectroscopy using samples prepared by chemical oxidation, and the analysis of the IVCT band by spectroelectrochemistry to evaluate the electronic interaction between the four redox centers of these molecular arrays.
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RESULTS AND DISCUSSION Synthesis and Characterization of Tetrametallic (LnMCC)(LnM′CC)bfc (5a−c). Originating from (HCC)(LnM′CC)bfc (LnM′ = Ru(Cp)(Ph3P)2 (4b), Os(Cp)(Ph3P)2 (4c)) and Fe(Cp*)(η2-dppe)Cl (2a) as well as from 4c and Ru(Cp)(PPh3)2Cl (2b), the appropriate tetrametallic mixed MM′Fe2 compounds (LnMCC)(LnM′CC)bfc (5a, LnM = Fe(Cp*)(η2-dppe), LnM′ = Ru(Cp)(Ph3P)2; 5b, LnM = Fe(Cp*)(η2-dppe), LnM′ = Os(Cp)(Ph3P)2; 5c, LnM = Ru(Cp)(Ph3P)2, LnM′ = Os(Cp)(Ph3P)2; bfc = biferrocene-1′, 1‴-diyl, (Fe(η5-C5H4)2)2; dppe = 1,2-bis(diphenylphosphino)ethane, Cp = η5-C5H5, Cp* = η5-C5Me5) were prepared using B
dx.doi.org/10.1021/om300050t | Organometallics XXXX, XXX, XXX−XXX
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which can be explained by the electron-withdrawing character of the transition-metal-containing entities [MLn]+ and [M′Ln]+, respectively. Comparison of the redox potentials of 5a−c with those of the symmetric complexes 3a−c indicates that the first oxidation of 5a,b is centered on the iron half-sandwich center and on the osmium termini of 5c. For complexes 5a,c, the second oxidation mainly concerns the ruthenium end. This is a general trend of this triad; for a homologous series of sandwich or half-sandwich complexes the first oxidation potential decreases in the order Ru > Os > Fe.20,21 An analogous trend is also known for redox potentials in corresponding metallocenes: i.e. Cp2Ru > Cp2Os > Cp2Fe.51,52 The substitution of ruthenium by iron often causes a cathodic shift in the redox potentials as large as 0.3− 0.4 V.20,21,34,51−53 In the present case, the first oxidation potentials of 5a and 3a differ by only 0.02 V, suggesting that the throughbridge electronic interaction between the remote termini is not strong. Changing from bimetallic (HCC)2bfc (1), which possesses two reversible well resolved redox events for the bfc unit, to tetrametallic MM′Fe 2 complexes 5a−c resulted in the appearance of four (5b) or even five reversible redox events (5a,c) (Figure 1). Comparison of the events found for 4b and 3b with those obtained for 5a indicates that in 5a the iron center of the terminal half-sandwich unit is oxidized at a more negative potential (−0.25 V) than the ruthenium(II) end (0.13 V, see Table 1). This potential is anodically shifted in comparison to 4b but is displaced in the opposite direction compared to 3b. Clearly, the iron fragment behaves as a more electron-donating group than the corresponding ruthenium entity. Comparison of the redox potentials found for 4c and 3c with those obtained for 5b indicates that the influence of the terminal iron center on the redox potential of the osmium-containing entity is negligible. In this case, the electron density is donated into the bfc system, as can be seen from the cathodic shift of the redox potentials for the bfc connectivity. In contrast, the CV of 5c shows that the oxidation at osmium is shifted to a more anodic potential. This confirms the thesis stated above concerning the withdrawing character of the different metal centers. The redox behavior of 5c
shift from 91.6 ppm in 2a to 112 ppm for 5a or 113 ppm for 5b and from 37.8 ppm for 2b to 50.4 ppm for 5c (for details see the Experimental Section). The 1H and 13C{1H} NMR spectra show no peculiarities, and hence they are not discussed here (for details see the Experimental Section). Cyclic Voltammetry. Cyclic voltammograms (CVs) were run from −0.4 to 1.4 V (vs a saturated calomel electrode, SCE) with compounds 5a−c (Figure 1). For comparison, molecules
Figure 1. Cyclic voltammograms of 5a (top), 5b (middle), and 5c (bottom) (10−3 M dichloromethane, 298 K, 0.1 M [nBu4N][PF6], platinum electrode, scan rate 0.100 V s−1).
(HCC)2bfc (1),49,50 (LnMCC)2bfc (LnM = Fe(Cp*)(dppe) (3a), Ru(Cp)(Ph3P)2 (3b), Os(Cp)(Ph3P)2 (3c)), and (HCC)(LnM′CC)bfc (4b,c) have also been studied (Table 1). The CVs of 5a−c show two well-separated and fully reversible redox events (ipa/ipc = 1) ranging between −0.3 and 0.2 V and corresponding to the stepwise oxidation of the terminal half-sandwich moieties MLn and M′Ln, respectively. In accordance with this assignment, the appropriate bis(alkynyl)biferrocene derivatives 4b,c featuring only one ruthenium or osmium M′Ln end group show, as expected, a unique wave at ca. 0.0 V. This event can safely be assigned to the respective [M′Ln]/[M′Ln]+ redox couple (M′ = Ru, Os). For the bfc connectivities of 3−5 two further redox events could be detected at more anodic potentials corresponding to the separate oxidation of the bfc ferrocenyl building blocks.33 These redox events are shifted to higher potentials in comparison to 1,
Table 1. Electrochemical Data for 3a−c and Related Compounds 1, 4, and 5a−ca compd
M
M′
[M2+]/[M3+] (ΔEp)
[M′2+]/[M′3+] (ΔEp)
1 3a
Fe
Fe
−0.23
3b
Ru
Ru
3c
Os
Os
4b
Ru
0.03 (0.09) 0.22 (0.1) −0.03 (0.09) 0.12 (0.05) 0.09 (0.1)
4c
Os
0.04 (0.1)
5a
Fe
Ru
−0.25 (0.1)
0.13 (0.1)
5b
Fe
Os
−0.27 (0.1)
0.04 (0.1)
5c
Os
Ru
−0.01 (0.09)
0.18 (0.09)
bfc/bfc+ (bfc+/bfc2+)
ΔEbfc
ref
0.47 (0.13) 0.85 (0.13) 0.49 (0.07) 0.77 (0.07) 0.74 (0.12) 0.89 (0.1) 0.62 (0.1) 0.79 (0.1) 0.65 (0.12) 0.89 (0.12) 0.60 (0.1) 0.82 (0.1) 0.60 (0.16) 0.90 (0.08) 0.55 (0.1) 0.77 (0.1) 0.65 (0.1) 0.88 (0.09)
380
32
280
32
150
33
170
33
240
33
170
33
300 220 230
In dichloromethane (0.1 M [nBu4N][PF6], 298 K, platinum electrode, scan rate 100 mV s−1), in V vs SCE, with [FcH]/[FcH]+ used as internal standard (E° = 0.46 V vs SCE).54 a
C
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OTTLE (optical transparent thin layer electrode) cell at ambient temperature.57 Figure 2 contains the spectra of
is situated between 3b and 3c (Table 1), which is the result of the great similarity of ruthenium and osmium, respectively. Comparison of the different potential separations found for the biferrocenyl core moiety shows the highest value to be 380 mV. Binding of different transition-metal entities to bfc resulted in a decrease of ΔE°′bfc. In fact, electronic communication between the iron atoms in the biferrocenyl moiety decreases. The largest effect was found for the ruthenium derivatives as can be seen from Table 1 and refs 51 and 52. Apparently, it seems that electronic communication between the two subunits of the bfc decreases. In fact, the origin of the variation of ΔEbfc is probably more complex, because the EPR data did not confirm this trend (vide infra). The CVs of 5a,c run at a scan rate of 0.100 V s−1 present anodic current peak ratios below unity (ipa/ipc < 1) for the two redox events centered on the bfc bridge, indicating that the oxidized species 5a[PF6]n and 5c[PF6]n (n = 3, 4) are not stable on the platinum electrode. Consequently, their lifetime is not long enough to allow the spectroscopic characterization at 20 °C. However, analysis of the peak currents in the CV of 5b, which does not contain a ruthenium moiety, shows that the anodic and cathodic currents are strictly identical, so that the electron-deficient species 5b[PF6]n (n = 3, 4) are apparently stable at the platinum electrode and their spectroscopic characterization constitutes accessible objectives as well as the species 5a−c[PF6]n (n = 1, 2). Note that the appearance of a fifth irreversible oxidation wave in the CVs of 5a,c which is not visible in the CV of 5b (see Figure 1) probably has its origin in the oxidation of a new electroactive species resulting from a chemical process in the vicinity of the electrode. IR Spectroscopy. In accordance with the symmetry of complexes 5a−c, the IR spectra are characterized by two νCC absorption bands in the range 2057−2077 cm−1 (Table 2). The
Figure 2. IR spectroelectrochemical measurement of 5b[PF6]n, from bottom to top: 0 < n < 1, −0.5 V < E < 0.25 V; 1 < n < 2, 0.25 V < E < 0.4 V; 2 < n < 4, 0.4 < E < 1.0 (V vs Ag/Ag+, 10−3 M dichloromethane, 298 K; 0.1 M [nBu4N][PF6]).
5b[PF6]n (n = 0−4). The spectra of 5a[PF6]n and 5c[PF6]n are given in Figures S1 and S2 (Supporting Information). Oxidation of 5a−c to 5a−c[PF6]2 proceeds via the observable intermediates 5a−c[PF6], which are characterized by two νCC bands. The lower energy band is shifted toward lower energy by ca. 90 cm−1, while the frequency of the higher energy band remained almost constant. Apparently, the oxidation is centered on one side of the molecule. To a limited extent, both sides of the molecule are involved in the oxidation of 5c into 5c[PF6]. The doubly oxidized species 5a−c[PF6]2 present a unique stretching band at a frequency intermediate between those found for the lower energy bands of 5a−c and 5a−c[PF6]. These two electron transfers seem to be reversible, indicating that the corresponding radical cations are thermally stable in solution on the time scale of a few hours. Furthermore, oxidation can reversibly continue to more cathodic potentials in the case of 5b. The progressive oxidation of 5b[PF6]2 to 5b[PF6]3 and 5b[PF6]4 is associated with a continuous decrease of the intensity of the band, making its analysis uncertain. This feature suggests that the bond order progressively increases as the oxidation proceeds. All processes are thought to be reversible, since after complete reduction spectra identical with those for the neutral species could be found. The IR data clearly indicate that the two termini of these molecules do not behave independently and electron transfer at one end is weakly sensed by the opposite terminus. However, the shift of the νCC band could result from a dominant interaction between the redox termini with the proximal bfc moiety rather than from direct interactions between the remote terminal half-sandwich groups. EPR Spectroscopy. In previous contributions, the homobimetallic mixed-valent complexes [((Cp*)(dppe)FeCC)2bfc][PF6 ] (3a[PF 6]), [((Cp)(Ph3 P) 2 RuCC) 2 bfc][PF6] (3b[PF6]) and [((Cp)(Ph3P)2OsCC)2bfc][PF6] (3c[PF6]) were subject to EPR studies.32,33 The spectra of the ruthenium and osmium complexes displayed the signature of a unique radical with an axial symmetry characteristic of a disubstituted biferrocenium radical, indicating that the SOMOs in these compounds possess a dominant biferrocenium character, as also found in Sato’s complex (Cp*)(dppe)FeCCfc, where the radical cation is localized on the ferrocenyl part.20 In contrast, in the iron series, the EPR spectrum of 3a[PF6] showed five g tensor components corresponding to the presence of two types of radicals.32 The narrower three g tensor components were ascribed to the pseudo-octahedral d5 low-spin [Cp*(dppe)FeIII] moiety, while the two broader lines were assigned to the bfc entity by comparison with the spectrum of [(HCC)2bfc][PF6] (1[PF6], see Table 3). It was noted that the relative
Table 2. IR νCC Vibrations for 5a−c, Related Complexes 1, 3a−c, 4b,c, and the Related Oxidized Speciesa compd
n=0
1[PF6]n 3a[PF6]n
2109 2075
3b[PF6]n 3c[PF6]n
2076 2057
4b[PF6]n
2074 2100 2069 2099 2075 2059 2077 2060 2075 2065
4c[PF6]n 5a[PF6]n 5b[PF6]n 5c[PF6]n a
n=1
n=2
ref.
2073 1988
1981
32
2056 1968
1976
33 33 32 32
2075 1967 2076 1968 2067 1980
1982 1976 1986
In cm−1.
frequencies are very close to those found for mono- and dinuclear compounds of the (Cp*)(η2-dppe)Fe−CCR and (Cp)(PPh3)2M−CCR (M = Ru, Os) type.28,49,50,55,56 It was of interest to explore the nature of the interactions occurring between the different metal termini via the bfc connectivity as oxidation processes occurred. The IR spectra of the chemically stable oxidized species 5a−c[PF6]n (n = 1, 2) were obtained using the spectroelectrochemical method in an D
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Table 3. EPR Data Determined in thf Glass at 66 K for 1[PF6], 5a−c[PF6], and 3a−c[PF6] M*/M compd 1[PF6] 5a[PF6] 5b[PF6] 5c[PF6] 3a[PF6] 3b[PF6] 3c[PF6] a
Fc-Fc
g1
g2
g3
gisoa
Δgb
2.328 2.33 2.30 2.343
2.041 2.04 2.15 2.047
2.010 2.01 2.02 1.998 not found not found
2.13 2.13 2.14 2.13
0.318 0.32 0.28 0.345
g∥
g⊥
Δgc
end/bridge
3.24 4.46 3.67 4.33 4.30 2.852 2.840
1.91 2.04 2.04 2.06 2.02 1.984 2.066
1.33 2.42 2.26 2.27 2.26 0.868 0.774
65/35 70/30 40/60 85/15 0/100 0/100
giso = 1/3(g1 + g2 + g3). bΔg = g1 − g3. cΔg = g∥ − g⊥; M* = Fe(Cp*)(dppe), M = M(Cp)(PPh3)2 (M = Ru, Os).
weight of the two components remained constant, when the EPR-active species were generated in situ using a variable amount of ferrocenium salt ranging from 0.2 to 1.0 equiv, indicating that the five tensors of the signal belong to a unique EPR-active species. An accurate simulation of the EPR spectrum (Figure 3) allows estimating that the spin density is shared between the iron ends and the biferrocenyl bridge in a ratio close to 85/15, respectively. To probe further the location of the spin density in the heterobimetallic mixed-valent (MV) systems [(LnMCC)(LnM′CC)bfc][PF6] (5[PF6]), the X-band EPR spectra were recorded at 66 K for these new MV compounds, generated in situ in DMF using 0.7−0.8 equiv of ferrocenium as oxidizing agent. The experimental spectra (Figure 3) show the characteristic signatures of the metal terminus and the biferrocenyl bridge. Spectral simulation allowed the determination of the two components and extraction of the g tensors given in Table 3. Interestingly, in the [Fe*]/[Ru] 5a[PF6] and [Fe*]/[Os] 5b[PF6] heterobimetallic mixed-valent systems the spin density is mainly delocalized on the metal ends, while the opposite situation was found in the [Ru]/[Os] 5c[PF6] MV complex. Nevertheless, it is interesting to note that in the case of the homobimetallic mixed-valent complexes EPR cannot evidence the contribution of the Ru and Os centers in the delocalization of the spin density. In the case of the [Ru]/[Os] heterobimetallic derivative the spin density on the metal ends is smaller than that on the bridge. This finding is in line with the observation of an EPR signal characteristic of the bfc moiety in the case of homobimetallic MV complexes 5b[PF6] and 5c[PF6] and suggests that the reorganization of the π system in the HOMO after the one-electron oxidation depends on the nature of the metal termini. Such a behavior has recently been observed in heterobimetallic complexes: i.e., [(bpy)(CO)3Re−(CCC6H4CC)−Fe(Cp*)(dppe)][PF6] and [(C7H7)(dppe)Mo− (CCC6H4CC)−Fe(Cp*)(dppe)][PF6].58,59 As observed in previous EPR investigations on mononuclear iron and ruthenium alkynyl complexes with the (Cp*)(dppe)M−CC backbone, the tensors of anisotropy Δg range from 0.48 to 0.51 for the iron derivatives53 and are significantly smaller in the ruthenium series.34 For the MV compounds considered here, the anisotropy is smaller than the values found in the mononuclear alkynyl derivatives, suggesting the active participation of the bridge in the delocalization of the unpaired electron. This is nicely corroborated by the continuous decrease of the Δg tensor with the ratio “end/bridge” given in Table 3. It is important to note that the g tensor of anisotropy (Δg = g∥ − g⊥) of the bfc moiety ranges between 2.26 and 2.42 for the three heterobimetallic mixed-valent complexes 5a−c[PF6]. As previously established by Dong and Hendrickson and already
Figure 3. X-band EPR spectra of 3a[PF6], 5a[PF6], 5b[PF6], and 5c[PF6] (from top to bottom, respectively) in frozen DMF (66 K) and simulated spectra illustrated as a sum of the metal termini and bfc contributions. For a given spectrum, from top to bottom: experimental, sum of the components, bfc component, and metal-end component.
found for the bis(iron) mixed-valent complex 3a[PF6], such a large tensor in disubstituted biferrocenium cations is diagnostic E
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of trapped mixed-valent systems at the bfc level.32,60−63 This means that the electron exchange rate between the fc units is slow on the EPR time scale (ke < 10−8 s−1). Considering the asymmetry of these radical cations, the localization of the unpaired electron is not surprising but nicely evidences the selfconsistency of the EPR data. Absorption Spectroscopy. As the tetranuclear complexes 5b[PF6]n present five reversible redox states (n = 0−4, vide supra), the UV−vis−near-IR spectra were collected using spectroelectrochemical methods between 400 and 3000 nm. The spectra of the five oxidation states were obtained with an electrochemical cell equipped with an OTTLE (Figure 4).57
Figure 5 for the series of complexes 5b[PF6]n (n = 2−4). The fitting parameters are summarized in Table 4. The experimental
Figure 4. UV−vis−near-IR spectra of 5b[PF6]n (bottom: 0 < n < 2, −0.5 < E < 0.37 and above from 2 < n < 3, 0.37 < E < 0.45; top from 3 < n < 4, 0.45 < E < 0.80 and above n > 4, 0.80 < E < 1.00, oxidation with decomposition), recorded in an OTTLE cell (V vs Ag/Ag+, 10−3 M CH2Cl2, 298 K, 0.1 M [nBu4N][B(C6F5)4]). Figure 5. Near-IR spectra of 5b[PF6]n and proposed deconvolution (from top to bottom: n = 2−4) recorded in an OTTLE cell (dichloromethane, 10−3 M, 298 K, 0.1 M [nBu4N][B(C6F5)4]).
The spectra of the thermally stable species 5a[PF6]n and 5c[PF6]n (n = 0−2) were also obtained in the same way and are given in Figures S3−S6 (Supporting Information). The UV sides of the spectra display very intense absorptions, corresponding to π → π* intraligand transitions. These bands are almost independent of the oxidation states of the complexes. In accordance with previous work, absorption bands are also observed at the limit between the UV and visible range and they can be assigned to MLCT transitions.32,33 As the oxidation proceeds, two bands arose in the visible region at ca. 570 and 620 nm to reach a maximum when the second oxidation is completed. Similar absorption bands were invariably observed in the spectra of mono- and polynuclear compounds containing the Cp*(η2-dppe)MIII−(CC) (M = Fe, Ru, Os) moieties.34,53,58 The spectra of the tricationic complexes 5a−c[PF6]3 are characterized by the disappearance of the bands around 600 nm and the appearance of a shoulder at ca. 700 nm. This behavior is consistent with the proposed assignment for the mono- and dicationic complexes: the bands disappear when the ligand is oxidized. During the progressive oxidation of 5a−c[PF6]3 to 5a−c[PF6]4 a new absorption appeared around 500 nm. The origin of this band, which was not observed in the spectrum of 3a[PF6]4, is not obvious. Note that the spectra of 4b[PF6]2 and 4c[PF6]2 display bands around 460 and 620 nm. The spectra of all oxidized species (n ≠ 0) here studied show absorptions in the near-IR range with several maxima. To facilitate their analysis, deconvolution of the spectra in the near-IR range was achieved using a combination of a minimum of overlapping transitions with Gaussian shapes. The fits are good enough to allow an almost exact overlay of the sum of the spectral components with the experimental spectra, as shown in
spectra of 5a[PF6]n and 5c[PF6]n (n = 1, 2) with deconvolution in Gaussian components are also reported as Supporting Information, and the corresponding fitting parameters are also given in Table 4. The near-IR spectra of the singly and doubly oxidized complexes present the same pattern, and the intensities of the maxima are roughly 2 times larger in the dicationic species than in the monocationic derivatives. With the exception of 5c[PF6]2, for which partial decomposition probably occurred, the fitting parameters nicely highlight this observation. Note that a similar observation was already reported for the symmetrical complexes 3a[PF 6 ] and 3a[PF6]2, respectively.32 In previous works we found that the two-state Hush model was well adapted to the electronic configuration of MV complexes in the Fe(Cp*)(η2-dppe) series, for which it has been shown that the SOMO-2 to SOMO-4 are largely metalcentered with strong d character.55,64 The near-IR spectra of these MV compounds were all characterized by three Gaussian curves of the same half-width and decreasing intensities as their energy increases. Meyer and co-workers established that these bands result from the coupling of the ligand field (LF) transitions with the MMCT.65 As already noted for 3a[PF6]n, the presence of two near-IR bands associated with the photoinduced electron transfer do not allow the use of the two-state Hush model to extract the parameters representative of the energetics of the electron transfer in the case of these complexes with a redox-active bridge connecting redox-active termini.32 F
dx.doi.org/10.1021/om300050t | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
Table 4. Fitting Parameters of the Near-IR Data for the Complexes 3a[PF6]n, 4b[PF6]n, 4c[PF6]n and 5[PF6]n in Dichloromethanea compd 3a[PF6]b
3a[PF6]2b
transition LF ILCT LMCT LF ILCT LMCT
3a[PF6]3b 3a[PF6]4b
4b[PF6] 4c[PF6] 5a[PF6] 5a[PF6]2 5b[PF6] 5b[PF6]2
LF LF LF ILCT/LMCT ILCT/LMCT ILCT LMCT ILCT LMCT ILCT LMCT ILCT LMCT
5b[PF6]3 5b[PF6]4 5c[PF6] 5c[PF6]2 a
ILCT LMCT ILCT LMCT
νmax (cm−1) (ε (M−1 cm−1)) 4000 (200) 5500 (900) 7600 (2300) 4000 (400) 5500 (1400) 7600 (4100) 5380 (1200) 8920 (2700) 4000 (
